1. Field of the Invention
This invention relates to a data acquisition system for acquiring acoustic information from objects submerged in a body of water and transmitting this information in the form of digital signals to data processing equipment. More particularly this invention relates to an array placed in a body of water for receiving acoustic signals from submerged objects, converting these signals to digital data words and transmitting these digital words through a single transmission line to remotely located data processing equipment.
2. Description of the Prior Art
Although prior systems have utilized multiplexing techniques for transmitting a plurality of analog signals through long wires to remote locations, these systems all have inherent disadvantages.
One such system includes a plurality of transducers which provide analog signals representative of a physical variable such as sound waves. Each of the analog signals produced by the transducers is amplified and transmitted down a separate transmission line to remote receiver circuitry, the separate transmission lines being incorporated in a cable. The receiver circuitry includes a multiplexer for sequentially sampling the analog signals and a A/D converter for transforming these samples to digital data acceptable by data processing equipment. Use of such an approach for acquiring acoustic information from an array of sensors towed by the cable and transmitting the information through the transmission lines to remote data processing equipment is impractical. It severely limits the number of sensors in the array and the length of the tow cable because of the high wire density and consequent weight of the cable.
Another approach which has been found usable for acquisition of information from a plurality of transducers and transmitting the information to a remote receiver requires the use of only a single transmission line. An A/D converter is connected to each transducer and the digital outputs from these converters are sequentially sampled and transmitted down the line. This approach, however, suffers the disadvantage of high cost and large array size because of the high number of A/D converters required.
The present invention overcomes the aforementioned disadvantages and provides an array including a high number of acoustic sensors, but requires only a single A/D converter and a single transmission line for transmitting information from the acoustic sensors to the remote data processing equipment. The present invention also minimizes size and wire density of the array. In addition, the timing of the multiplexing devices is controlled in such a manner that multiplexing transients are not communicated to the data processing equipment.